The performance of sexual behavior in mammals seems to be regulated by two independent motivational substrates, arousability and orientation. Arousability determines the threshold for sex behavior, while orientation influences the choice of the sex partner's gender. While a great deal is known about the mechanisms that regulate arousability, much less is known about the mechanisms that contribute to orientation. A major reason for our ignorance of the causes of sexual orientation is that most animal models that have been described for this phenomenon have been rejected as unsatisfactory. The proposed research represents an attempt to explore some biological factors which may affect a newly described, more satisfactory animal model. The animal model for sexual orientation which will be explored involves the attraction that male and female mice (Mus musculus) exhibit for conspecific urinary odors. The proposed research will examine whether the sexual dimorphism in mouse urinary preferences is organized by the neonatal effects of gonadal hormones. A variety of different strategies will be employed, including neonatal and adult gonadectomy, infant and adult hormone replacement, neonatal blockage of androgen and estrogen receptors, and neonatal blockage of the conversion of testosterone to either estrogens or 5-alpha reduced androgens. In addition, an attempt will be made to map the neonatal sensitive period for hormonal action. This research has a high probability of providing important information about the physiological substrates of mouse sexual orientation.